The present disclosure relates to an integrator, and more particularly to an integrator suitable for a continuous-time ΔΣ modulator, etc.
Oversampling A/D conversion, which is in widespread use in the front-end of communications equipment, conversion of audio signals, etc., is a circuitry technology essential for the current communications, video, and audio signal processing circuits. One type of oversampling A/D converters is a continuous-time delta-sigma (ΔΣ) A/D converter (CTDS-ADC) having a continuous-time filter (see Richard Schreier and Bo Bang, “Delta-Sigma Modulators Employing Continuous-Time Circuitry,” IEEE Transactions on Circuits and Systems—I: Fundamental Theory and Applications, Vol. 43, No. 4, April 1996 and Xuefeng Chen et al., “A 18 mW CT ΔΣ Modulator with 25 MHz Bandwidth for Next Generation Wireless Applications,” IEEE 2007 Custom Integrated Circuits Conference, 2007, for example).
In a general CTDS-ADC, an input signal passes through n-cascaded integrators (continuous-time filters) and then is quantized by a quantizer. The digital output of the quantizer is fed back to the n integrators after being converted to an analog current signal by n D/A converters. In the CTDS-ADC, having no switch in its analog circuit portion, the voltage can be reduced. In addition, it is unnecessary to place a prefilter that is normally necessary when a sampling filter is used. Having these features, the CTDS-ADC is suitable for applications to communications systems, and thus recently application development and research have been actively conducted.
The inventor of the present disclosure has found the following on the conventional CTDS-ADC. In order to improve the resolution and SN performance of the CTDS-ADC, the filter order for removal of quantization noise must be increased, and this necessitates the number of operational amplifiers corresponding to the increased filter order. That is, to improve the performance of the CTDS-ADC, a number of operational amplifiers must be used. However, increase in the number of operational amplifiers will increase the circuit scale and the power consumption, causing a bottleneck in improving the performance of system LSIs applied to mobile communications equipment, etc.